- Research Article
- Open Access
A Family of Key Agreement Mechanisms for Mission Critical Communications for Secure Mobile Ad Hoc and Wireless Mesh Internetworking
© Ioannis G. Askoxylakis et al. 2011
- Received: 30 June 2010
- Accepted: 17 September 2010
- Published: 23 September 2010
Future wireless networks like mobile ad hoc networks and wireless mesh networks are expected to play important role in demanding communications such as mission critical communications. MANETs are ideal for emergency cases where the communication infrastructure has been completely destroyed and there is a need for quick set up of communications among the rescue/emergency workers. In such emergency scenarios wireless mesh networks may be employed in a later phase for providing advanced communications and services acting as a backbone network in the affected area. Internetworking of both types of future networks will provide a broad range of mission critical applications. While offering many advantages, such as flexibility, easy of deployment and low cost, MANETs and mesh networks face important security and resilience threats, especially for such demanding applications. We introduce a family of key agreement methods based on weak to strong authentication associated with several multiparty contributory key establishment methods. We examine the attributes of each key establishment method and how each method can be better applied in different scenarios. The proposed protocols support seamlessly both types of networks and consider system and application requirements such as efficient and secure internetworking, dynamicity of network topologies and support of thin clients.
- Elliptic Curve
- Mesh Network
- Wireless Mesh Network
- Mesh Router
- Elliptic Curve Cryptography
Consider a disaster situation, such as an earthquake, a flood, or a terrorist attack, where the commercial network infrastructure is destroyed or out of order. The objective of the rescue workers is to set up quickly, efficiently, and easily a wireless network among them in order to help in a coordinated way the affected population. Their goal is to interconnect all their computing and communication devices, in a way that will enable them to share all necessary information securely, in a way that they could be sure that possible "high tech" terrorists/attackers in their range will not be able to disrupt or intercept the rescue efforts.
In real disaster scenarios, emergency response does not take place all at once. We usually observe an escalation in the presence of several groups of rescue workers and prioritized escalation of their efforts. In the beginning, we usually observe ad hoc groups working as independent teams that arrive at place independently. These teams gradually become part of coordinated action by a central disaster management entity, which requires more time to arrive at place, set up its infrastructure, and become operational. Approaching this scenario from a networking perspective, a sufficient approach would be the support of the initial groups of rescue workers by communication devices with mobile ad hoc networking capabilities. In this respect, an efficient networking solution for the support of the central disaster management entity would be the employment of adaptive, self-organized networks with advances networking capabilities, and redundant characteristics like wireless mesh networks. Seamless interworking of both types of networks would be a key requirement for such a scenario.
Security is a primary concern for providing protected communications in such environments where there is no available communication infrastructure and where networks of varying types and sizes must be established quickly and dynamically. Moreover, there might be situations where potentially large numbers of rescue workers, potentially from multiple government services or even nations must cooperate and coordinate their efforts in areas where natural or man-originated disasters have damaged or set temporarily out of order part or the entire telecommunication infrastructure. The unique nature and characteristics of mobile ad hoc Networks and wireless mesh networks make them ideal networking solution to the above situations. At the same time, their nature and characteristics pose a number of nontrivial challenges to their security design, architecture, and services.
In both MANETs and wireless mesh networks, like in any other type of network, trust cannot be created among the network nodes without the existence of predefined prior known information available to all nodes beforehand. This special kind of information is necessary in order to build trust between all participating nodes. A network is established among the existing nodes, if from this preexisting information known to all network nodes, we reach a state where a common session key is agreed among the nodes.
The technical goal is to make sure that no other entity outside the group should be able to gain access within the new network. However, since neither a certification authority nor a secure communication channel exists, potential attackers have the ability to eavesdrop and modify exchanged messages transmitted over the air. Additionally, since no central identification authority is present, group member impersonation is easy, jeopardizing the security of the whole system.
Considering all these issues, the main challenge that arises is the setting up of a wireless network where the legitimate members of a group will be able to establish a protected wireless network. Moreover, in the case where a new node arrives at place, desiring to become a member in an already established group, joining, without delaying or even intercepting the existing group, is also challenging. The case where a group member is captured by the enemy, and therefore the group key is compromised is also part of the considered scenario. All the above considerations become even more challenging for the mobile ad hoc/wireless mesh internetworking scenario examined in this work.
The rest of the paper is organized as follows. In Section 2, we describe the system model. In Section 3, we describe the adversary model, and in Section 4, we present the security requirements. In Section 5, we present a review of the related work concerning two-party and multiparty key agreements, and we give a brief introduction on weak to strong authentication and the elliptic curve theory. In Section 6, we describe specific multiparty key agreement protocols and particularly, the BCC, the FCC algorithms, and the tetrahedral approach and examine their properties. Finally, in Section 7, we conclude with suggestions for future work.
We assume that the mesh routers belong to multiple operators, and they cooperate for providing aggregate networking services to all of their mesh clients. In the disaster management scenarios, we consider as different operators different teams of rescue workers (firemen, policemen, etc.). Their cooperation model, which falls out of the scope of this paper, can be based on simple on field agreements or on business agreements similar to roaming agreements in the case of cellular networks. Mesh clients are mobile computing devices (smart phones, PDAs, netbooks, etc.) operated by customers that can be associated with one or more operators by contractual means.
The mesh network provides various services to its clients like Internet access, real-time communications within the mesh network, and so forth. In this model, the mesh network is also designed to provide QoS applications with client mobility support. This way mobile mesh clients can perform seamless handovers between access points.
In parallel to the wireless mesh architecture, in our system model, we have the presence of independent mobile ad hoc networks as shown in red in Figure 1. A MANET is a type of network, which is typically composed of equal mobile hosts that we call nodes. When the nodes are located within the same radio range, they can communicate directly with each other using wireless links. This direct communication is employed in a distributed manner without hierarchical control. The absence of hierarchical structure introduces several problems, such as configuration advertising, discovery, maintenance, as well as ad hoc addressing, self-routing, and security .
In our internetworking model, a MANET node can be also considered as a mesh client and can perform seamless handovers between access points of the mesh network or between the MANET and the mesh network.
As usual, the first step in the identification of security requirements is the understanding of the potential attacks against the system. This understanding is summed up in the following adversary model that describes the classes of attackers, their objectives and their means to attack the network.
Taking into account the system model of mobile ad hoc and wireless mesh internetworking, the following types of attackers are identified.
These are attackers that have no legitimate access to the MANET or the wireless mesh network but they have appropriate equipment to use the wireless medium and interfere with the operation of the network protocols.
These are legitimate node devices that have legitimate access to the MANET and/or the wireless mesh network services and they have been compromised by attackers (e.g., by stealing a device or by capturing a legitimate user in the field). The attackers have the knowledge to modify the behavior of these nodes and try to take advantage of this in order to interfere with the operation of the network or to gain illegal access to its services.
Dishonest Network Nodes/Clients
They are misbehaving end users that while they have legitimate access to the wireless networks and some or all of the network services, they try to take advantage of this in order to gain illegal access to services that are not subscribed to, or to obtain higher QoS in services that they are already subscribed.
Dishonest Network Operators
They are operators of the mesh infrastructure that do not honestly keep to cooperation agreements.
Next, we identify the following main objectives of attacks.
The objective of this type of attack is to degrade the QoS provided by the mesh network and/or the MANET or even to completely disrupt the provided services. This is an objective of external adversaries.
Unauthorized Access to Services
This objective is mainly related to external adversaries and dishonest clients. Common services include internet access and real-time communications.
Unauthorized Access to Network Client Data and Meta-Data
Network client data are the messages exchanged in a service session and the corresponding objective is the violation of the confidentiality of the client whereas meta-data is information related to the client's location and service usage profile and the objective is the violation of the privacy of the client. Primarily, this objective is related to external adversaries and dishonest network operators.
Fraudulent Improvement of Operator Profile
This could be the objective of dishonest operators that may mount attacks on the mesh network or specific network operators/competitors participating in the network in order to gain some advantage over them. This can be achieved either by reducing or destroying the reputation of the competitors, or by spuriously increasing their own reputation.
There is a broad range of attack mechanisms that can be used and combined in order to reach the goals described above. However, most of these mechanisms fall into either one of the following two categories.
(i)attacks on wireless communications (including eavesdropping, jamming, replay, and injection of messages, and traffic analysis).
(ii)compromising existing nodes (typically by physical tampering or logical break-in). The behavior of the fake or compromised nodes can be arbitrarily modified in order to help to achieve specific attack objectives. In such a scenario, the underlying security depends on the size and the randomness of the chosen password. However, the larger the password gets the more difficult it is to memorize and use. Moreover, since the response time is vital during emergency operations, the use of large passwords can be proved inconvenient. Therefore, the use of short, user-friendly passwords is an essential requirement;
(iii)setting up fake mesh routers or compromising unattended existing mesh routers.
It is broadly known that security mechanisms cannot create trust . The members of a team that wish to establish a group know and trust one another physically. Otherwise, they would never be able to achieve mutual trust regardless of the authentication mechanism used. Our goal is to exploit the existing physical mutual trust and create a secure group of communication for both types of networks that would operate in a seamless manner.
An efficient solution to this direction, without adding new requirements like the use of dedicated hardware (i.e., smart cards), would be a password authentication mechanism. A simple approach of a password-based authentication scheme could be the use of sufficiently large and randomly generated data strings employed as passwords. In such a scheme, all nodes could agree on a password and achieve mutual authentication supported by a trivial authentication protocol.
In such a scenario, the underlying security depends on the size and the randomness of the chosen password. However, the larger the password gets the more difficult it is to memorize and use. Moreover, since the response time is vital during emergency operations, the use of large passwords can be proved inconvenient. Therefore, the use of short, user-friendly passwords is an essential requirement.
The use of short passwords provides weak authentication since the password selection set is quite limited, and thus the corresponding authentication procedure is vulnerable to dictionary attacks . Therefore, we need an authentication protocol that will lead to a reasonable degree of security even if the authentication procedure has been initiated from a small, weak password.
Below, we outline the main security requirements of the proposed architecture.
Weak-to-Strong Password-Based Authentication
Use of an authentication scheme that will lead to a reasonable degree of security although the authentication procedure has been initiated from a small, weak password.
Only the entities that hold the correct password will eventually become members of the network.
Even if a malicious partner manages to compromise a network entity in a later phase, he will still be unable to participate in the already existing network.
Contributory Key Establishment
The network is established when a session key is generated and agreed among all network nodes. The session key should be generated throughout in a contributory manner, by all participating entities.
Security Architecture for Thin Clients
In both types of networks, there are mobile devices/clients with limited processing power and energy consumption. The cryptographic algorithms used for authentication and key agreement should add minimal computational overhead.
Rare Key Reestablishment
Session key refreshments should be performed as rare as possible, since during every new key reestablishment session the network is unavailable for node communications.
Unified Security Architecture for Combined MANET-Mesh Secure Internetworking
The proposed key agreement mechanisms should apply in both types of networks, without requiring any network-specific adjustments.
5.1. Password-Based Key Exchange
This message includes 's id in clear text.
If the challenge response protocol has been successfully deployed, then the authentication process is successfully accomplished and both parties proceed, using the symmetric cryptosystem and the quantity as the session key. However, this protocol has a major drawback. That is, the creation of the common session key has taking place with unilateral prospective, that is, only by the entity that first initiate the whole procedure. Thus, the key agreement scheme is not contributory.
where is the Public key of . is the random quantities generated from , and is the session key produced according the formula . is an n-input one way function and is a public hash function.
5.2. Password-Based Diffie-Hellman Key Exchange
5.2.1. Two Party Key Exchange
picks a random number, calculates , and sends , to ; entity 's id is sent in clear text.
The session key is derived from this value by selecting a certain number of bits. Finally, a random challenge, is generated. Then, transmits
uses to decrypt . From this, quantity is calculated; is in turn used to decrypt . then generates a random challenge . sends
decrypts to obtain and verifies that it matches the original message.
5.2.2. Elliptic Curve Diffie-Hellman
The original Diffie-Hellman algorithm is based on the multiplicative group modulo . However, the elliptic curve Diffie-Hellman (ECDH) protocol is based on the additive elliptic curve group, and it is desribed below. We assume that two entities , have selected the underlying field, or , the elliptic curve with parameters , , and the base point . The order of the base point is equal to . Also, we ensure that the selected elliptic curve has a prime order, in order to comply with the appropriate security standards [9, 10].
Entity selects an integer,
Entity selects an integer
computes . The pair consists of 's public and private key.
computes . The pair consists of 's public and private key.
Quantity is now the common shared key between and . Moreover, it can also be used as a session key. Quantity is the order of the base point .
5.2.3. Password-Based Elliptic Curve Diffie-Hellman
The efficiency of elliptic curves in terms of security and calculation efficiency has been extensively discussed [10, 11, 12, 9a, 14]. Therefore, their employment in the password-based Diffie-Hellman process would significantly accelerate the key establishment procedure. The importance of this enhancement becomes even greater in the case of an emergency situation, where all actions should be performed in the fastest and more secure possible way consuming limited computing power.
to ; entity 's id is sent in clear.
picks a random number and calculates . also uses the shared password to decrypt and calculates
The session key is derived from this value, perhaps by selecting certain bits. Finally, a random challenge is generated. transmits
uses to decrypt . From this, is calculated; is in turn used to decrypt . then generates its own random challenge . sends
decrypts and verifies that is correct. sends
decrypts to obtain and verifies that it matches the original message.
5.3. Efficient D-H-Based Multiparty Key Exchange
5.3.1. d-Cube Protocol Overview
For key establishment procedures in multiparty networks like MANETs and mesh networks, where several entities are involved, multiparty authentication protocols should be applied. A lot of research has been done in this direction. Becker and Wille  presented a method very efficient in terms of number of authentication rounds. According to this method, also known as -cube protocol, all entities planning to participate in a network are initially arranged in a -dimensional hypercube. Each potential network entity is represented as a vertex in the dimensional-cube, and it is uniquely assigned a -bit address. The addresses are assigned in a way so that two vertices connected along the i th dimension differ only in the i th bit. There are vertices each of which are connected to other vertices.
5.3.2. DH d-Cube
Assume that there are entities seeking to establish an ad hoc non infrastructural network. During the first step, each entity is assigned to a vertex in the hypercube, and it is given a unique -bit address. The deployment of the address arrangement is out of the scope of this paper and will not be examined. The key establishment protocol is illustrated within rounds. In every single round the entities are paired together, according to a specific procedure, and the Diffie-Hellman key exchange is performed. These pairwise operations are performed in parallel during every round. For example, during the i th round of the protocol a node with address performs a two party Diffie-Hellman key exchange with the node whose address is . So, in the i th round there will be pairs of groups, each group consisting of nodes. By the time the d th is completed, a contributory session key will have be created. Next, we will present graphically the 2-d and 3-d cases.
In , the authors incorporate the password-based authentication into the cube protocol. This is achieved by using the four-move two-party password authenticated Diffie-Hellman protocol for pairwise exchanges in each round of the -cube protocol.
must not match two nodes to the same partner in a given subround.
must not select the same partner twice.
computation and transmission of all outgoing messages,
reading of all waiting messages and state transition accordingly.
Every node has a three bit address and a three bit mask, and it is labeled from to . Its key contribution is represented by the corresponding lowercase letter.
Labels next to the arrows indicate the nodes that have already contributed, directly or indirectly, to the key. Suppose that player (with address 110) is unsuitable (unavailable or does not know the password). In round 1, player (111) will initiate the procedure of selecting as a partner the node whose address is 110 and mask 000.
The exchange attempt with fails and the mask is already $000$. So, does nothing in this round. In round 2, $100$ will start with $110$ as candidate address and 001 as mask. The first recursive call will try $110$ as candidate address and $000$ as mask and will fail. The second recursive call will try $111$ as candidate address and $000$ as mask and will succeed. Similarly, in round 3 and Figure 4, node ($010$) starts partner finding with $110$ as candidate. The work in  also considers the case, where the total number of nodes is not more than , while the number of the faulty nodes is for some . The of them are located in a single -cube , and the rest of them in a -cube .
The number of subrounds required in rounds from to where are at most per round. This is because in each of those rounds, there is always one subround with faulty partners. The same faulty node may select using each of the faulty partners in sequence before being able to complete its round exchange, thus resulting rounds. Since there is no other subcube with more faults, is the maximum number of subrounds required.
Thus, the total number of communication rounds required to complete the exchange is . This case incurs the maximum possible number of subrounds in the worst case during round 1 to round.
In this section, we describe a family of key agreement protocols initially employed only in MANETs and the way that can be implemented in a MANET/mesh internetworking system.
In the approach described in Section 5, the only way to obtain a common session key when one or more nodes depart from the established MANET is to start over the algorithm from the very first step. Furthermore, there are no intermediate session keys stored between nodes that are still part of the network, which could be proven to be useful for node-to-node communication, when global session key is no longer valid due to network reform. Such approaches tend to be sufficient in relatively stable networks, where their topology does not change frequently. However, when network topology dynamicity increases, creating new global session keys very often is not the optimum solution.
The following algorithms propose efficient means for creation and use of intermediary session keys at the same time with the creation of the global network key, which can be used both for subgroup communications and as intermediate step for key refreshment of the global session key, without the obligation to restart the group key agreement.
6.1. The Body-Centered Cubic (BCC) Algorithm
The body-centered cubic algorithm  is a cryptographic key agreement algorithm that initiates from a tree-arrangement of 3-d cubes; it is based on the aggressive 3-d cube algorithm and employs the body-centered cubic (BCC) structure for the dynamic case. For simplicity purposes, in the rest of the paper, each bond in 2-d or 3-d space corresponds to a two-party password-based elliptic curve Diffie-Helman key exchange, as described in Section 5.2.3.
6.1.1. Initial Node Arrangement
6.1.2. Initial (Static) Key Agreement
Next, after the initial 3-d arrangement, BCC creates a common network key. In the proposed system this is done in two steps.
During the first step, the leading nodes perform a 3-d aggressive cube algorithm and they create a global session key. In the second step, every group performs a 3-d aggressive -cube and establishes a simple-cube session key. During the simple-cube key generation, the leading nodes transmit the global session key that they have already established in step 1 to the remaining seven nodes of the group. After the second step, every node has a contributory simple-cube session key for the cube that is part of, and the global session key of the entire network .
In the first step, nodes of cube , of cube , of cube , and of cube are elected as leading nodes of the corresponding cubes. Since they are four, they perform a 2-d aggressive algorithm, and they establish a global session key . If there are than four and equal or less than eight 3-d cubes, their leaders should perform a 3-d aggressive cube algorithm. In this case, the leading nodes can use the first two digits of their addressees as a 2-d address for the 2-d aggressive algorithm, that is, , , (10), and . If other nodes are elected as cube leaders due to communication constraints, they should be addressed in a separate way than the one employed in their 3-d cube (second addressing is required).
Once the global session key of a group is established the cubes perform a 3-d aggressive -cube, and they establish the simple-cube session key . The final step is that each cube leader broadcasts the global key encrypted with the simple-cube key to the rest of the cube members. At the end of the protocol, every node has a simple-cube session key for secure communications among nodes of simple-cubes, and a global session key , for the entire group (mesh or MANET).
6.1.3. BCC for the Dynamic Case
Above, we described the initial arrangement-addressing of nodes and the generation of a global and of simple-cube session keys. These keys are static, since if there is a need to add new nodes to the network, the key generation procedure must be repeated. Here, we describe an efficient method for dynamic key generation every time new nodes arrive to or depart from our network. The proposed dynamic algorithm is based on the body-centered cubic structure, and we call it BCC algorithm.
Case 1 (Adding nodes to an established network).
The BCC algorithm operates in the following way: assume that a group has been established as previously described. Assume that one simple cube of this network is depicted in Figure 7(a). At some point of time, seven new nodes arrive and request to join the network. If the number of the new arriving nodes m is a multiple of 8, that is, , then in groups of 8, they perform aggressive cube algorithms and each group elects a leader that will contact leaders of new groups and leaders from the established network in order to create a new global session key. If , then we will have new groups of 8 nodes where is the integer part of and the number of the remaining nodes where while the groups of 8 nodes will perform new aggressive cube algorithms, the remaining node will attach to an existing cube of the network in the following way.
The first four new nodes are assigned addresses that correspond to the center of the existing cube the centers of the right, upper, and front cubes as shown in Figure 7(c) while the last three are assigned addresses that correspond to the centers of the left, back, and down cubes as shown in Figure 7(e). Keep in mind that the six neighboring cubes do not exist as network cubes; they are used as geometrical objects for demonstration purposes of the BCC algorithm. The first four new nodes (the body-centered cubic node and three central nodes of neighbor cubes) create a new cube with four nodes of the preexisting network cube as shown in Figure 7(d), and they perform a new aggressive cube algorithm. The latter 3 new nodes together with the body-centered cubic node (the node assigned to center of the existing cube of the network that has already participated in the previous new aggressive cube), and the remaining four nodes of the preexisting cube of the network perform a second new aggressive cube algorithm as shown in Figures 7(e) and 7(f).
Case 2 (Extracting nodes from an established network).
6.2. The Face-Centered Cubic (FCC) Algorithm
The 6 new nodes together, with nodes and that contributed to the initial cube, create a new cube and perform a new (second) aggressive 3-d cube algorithm. This way the inner cube creates a second common session key. After the setup of the second session, key nodes and hold both session keys corresponding to both cubes. This privilege makes nodes ( and leading nodes for the established network since any communication between black and grey nodes should pass through them. If we wish to avoid this hierarchy in our network, during the set up of the common session key within the inner cube, nodes and propagate the common session key of the initial (black) cube to the new nodes. This way the first session key can be used by all nodes to communicate securely with each other, while the second can be used for the secure communication of the internal (grey) cube.
In the hierarchical model, where every cube has its independent session key, key refreshment due to departing nodes is easy. As soon as a node is leaving the network, the rest nodes of the common cube, perform a new aggressive 3-d cube algorithm and create a new session key. In case the leaving node is belonging to two consecutive cubes, a new aggressive 3-d cube algorithm is performed automatically to both cubes.
In the case where the previous session keys belonging to previous cubes are forwardly distributed to the next cubes, the key renewal should be performed to all previous cubes. This appears not to be a desired feature, since if there is a departure in the last cube all previous stages/cubes will be affected. However, this can be also avoided if the set up is a combination of the two solutions. Periodically the key forwarding method is interrupted by the hierarchical solution. This way, we create isolated groups of concatenated cubes and any necessary key refreshment is bounded within these groups.
6.3. The Tetrahedral Approach
In the tetrahedral approach , in contrast to [16, 20] all intermediate two-party ECDH keys are stored by each node. This way, when a global session key has been created, every node in the d-dimensional cube maintains also a list of all the two-party ECDH keys that has created with every of her closest neighbors during the global key generation.
In the BCC and FCC approaches as described above, as soon as the initial phase of the proposed algorithm is completed, each node posses the global session key, and the two-party keys with her closest neighbors. At that point, if a node leaves the network, the global session key should be refreshed and in the meantime, secure communications would be only available between couples of closest neighbors that have already established two-party keys during the creation of the latest session key, which is no longer valid. This way, communication between distant neighbors, is only available through multihopping between nodes that, in couples, maintain valid two-party keys. This would add another requirement for routing metric information maintenance by all nodes, in order to serve each other to find the right secure path in the network. Solution to this direction is provided by the proposed integration of tetrahedral group key agreement in the existing cubes. The proposed structure covers both cases ( -cube and aggressive -cube) and takes place right after the creation of the global session key.
Below, the procedure is explained in detail.
This is due to two reasons: every node has a two party key with all nodes of the cube except the most distant node. For example, has two party keys with every node of the cube except node . Besides the group session keys among every 4 nodes forming a square edge of the cube, the four triangles of each internal tetrahedron shares a common session key, since the ECDH key exchange this time is the party D-H key exchange instead of two-party in all other cases.
Let us provide an example to demonstrate the attributes of the proposed algorithm. The reference node for this example will be node . During the initial 3-d or aggressive 3-d algorithm, node creates three two-party keys with nodes , , and and the global session key of the cube. During the tetrahedral algorithm, node creates three two-party keys with nodes , and . This way, node maintains two-party keys with all nodes of the cube except .
If any of its first and second order closest neighbor leaves the network, during the global key renewal node will be still able to communicate with all expect one of the remaining nodes. However, this distant node (in the example node ) belongs to the other tetrahedron and has keys for communication with the remaining nodes.
In another case, if decides to leave the network, its distant nodes, belonging to the left tetrahedron, do have keys for secure communications (a session key for the hole tetrahedron plus the two party keys among any pair of them), while the remaining three nodes , , and of the right tetrahedron, besides the two-party keys, they have a three-party key established among them during the last stage of the tetrahedral key agreement algorithm. Therefore, when a node leaves the network, the remaining nodes have all the two-party session keys, a four-party session key of the tetrahedron that did not change formation, and a three-party session key of the triangle composed of the three remaining nodes of the tetrahedron that the leaving node was part of.
6.4. Analysis of the Algorithms
All proposed algorithms integrate elliptic curve cryptography together with the password-based DH key exchange protocols and extend this approach to the multiparty case where new network nodes arrive or existing network nodes depart from the network.
The static multiparty case is considered and after careful examination the aggressive -cube algorithm is selected in the 3-d case. The algorithms are extended versions of the -cube algorithm proposed in , with additional features that enhance the resistance against the dictionary attacks . The basic idea behind the algorithms' design is to isolate the faulty nodes in the earliest possible stage. We managed to reduce the interaction with the faulty nodes and therefore minimize the exposure to dictionary attacks and other types of attacks. However, the aggressive behavior of the algorithm may lead to isolation of even legitimate members due to reasons such as the loss of commutation signal or the typo errors of the initial password during the authentication procedure. To overcome these kind of problems and to provide a more dynamic and robust solution, we propose the body-centered cubic (BCC) as well as the face-centered cubic (FCC) and the tetrahedral algorithm.
The BCC and the FCC algorithms can be considered as a series of different aggressive cube algorithms, each one performs for a different set of network nodes. In terms of two-party ECDH procedures, the BCC and FCC complexity is not higher than any of the single aggressive cube algorithms. This is because within a single two-party ECDH procedure of each algorithm, three ECDH procedures run simultaneously, each one corresponding to a different round of the aggressive cube algorithm.
The tetrahedral algorithm has significant differences compared with the BCC or the FCC algorithms. In this algorithm, all intermediate two-party keys are stored locally by the nodes and may be used during the network reformation process. Moreover, three-party keys are stored, providing the ability for secure subgroup communications among nodes that share the corresponding keys.
Depending on the mobility pattern and the reformation speed of the network, the aggressive -cube algorithm is more efficient for almost static networks, the FCC approach would be more convenient for small-medium network reformation, BCC for medium-high network reformation and the tetrahedral algorithm would better facilitate high dynamicity in network reformation.
Our research was motivated by the need to establish fast, reliable, efficient, and secure group communications without relying on preexisting infrastructures. The actual operational environment and the varying nature of the established networks impose further key issues (e.g., the ability to add or subtract nodes depending on operational and security considerations) that need to be taken into account.
We have reviewed existing proposals around two-party or multiparty authentication and introduced new key establishment methods. Our proposal overcomes some of the main issues (such as rapid deployment, accuracy, and dynamic and robust behaviour) of existing solutions and operational environments. The proposed solutions introduce the use of elliptic curve cryptography in such a scenario. ECC computations require less storage, less power, less memory, and less bandwidth than other systems. This allows implementation of cryptography in constrained platforms such as wireless devices, handheld computers, smart cards, and thin-clients. For a given security level, elliptic curve cryptography raises computational speed and this is important in ad hoc networks, where the majority of the clients have limited resources.
The proposed algorithms meet all security requirements according the initial specification and provide differentiated solutions depending on the network reformation dynamicity. They are designed to support in a uniform approach both MANETs and wireless networks, taking into account a broad range of application requirements including secure internetworking and dynamicity of network topologies.
The proposed algorithms leave several open issues for future work. Formal analysis is necessary. The incorporation of several new password-based key agreement protocols, which do not require the use of asymmetric encryption, is a challenging consideration. Studying in detail the dynamic case, where the network topology is rapidly changing, would be very interesting, especially for indentifying the thresholds of network reformation that should be employed for algorithm switching.
This work was supported in part by the European Commission in the 7th Framework Programme through project EU-MESH (Enhanced, Ubiquitous, and Dependable Broadband Access using MESH Networks), ICT-215320, http://www.eu-mesh.eu/.
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